1. Field of the Invention
This invention, in its preferred form, relates to apparatus for laser machining, and in particular laser welding, of at least first and second work pieces. More specifically, this invention relates to plural computer controls for controlling the laser machining operations on the first and second work pieces including the control of the movements of each work piece so that they are accurately positioned with respect to a laser beam, whereby a sequence of laser machining steps may be carried out on each work piece under the control of its associated computer. More particularly, this invention relates to apparatus for welding the elements, i.e. grid spacers, of a nuclear fuel rod grid.
2. Description of the Prior Art
The precision laser welding apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assemble 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Through not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grids straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attach the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grid straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 48 and tab portions 50a and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 of Patterson et al., assigned to the assignee of this invention, disclose similarly configured fuel rod grids of the prior art. Each of these patents discloses a fuel rod grid wherein the inner and outer grid straps are made of a suitable metallic alloy such as Inconel, and the above identified interconnections are effected by furnace brazing. However, the zirconium alloy Zircaloy is known to have the desirable characteristics of a low neutron absorption cross section which allows for more efficient use of the nuclear fuel in the utility operation and therefore allows for a longer elapsed time between refueling by the replacement of the nuclear fuel bundle assemblies. In particular, fuel rod grids made of Zircaloy have a lower absorption rate of the neutrons generated by the fuel rods than that absorption rate of straps made with Inconel. The making of the grid straps of Zircaloy requires at least several changes in the assembly of the fuel rod grids. First, it is necessary to make the slots, whereby the inner grid straps may intersect with each other, of looser tolerances in that grid straps made of Zircaloy do not permit a force fitting thereof, i.e. to be hammered into position, but rather require controlled fit-up to allow "push-fits" of the intersecting grid straps. In addition, Zircaloy grid straps may not be brazed in that heating Zircaloy to a temperature sufficient to melt the brazing alloy would anneal the Zircaloy, resulting in a loss of mechanical strength.
U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body of prior art disclosing automated laser welding apparatus in which the position of the work piece, as well as the welding process, is controlled by a digital computer. Kerth shows the control of laser beams while controlling the work piece as it is moved from side to side along an X axis, horizontally forward and backward along a Y axis and vertically up and down along a Z axis. Typically, pulse driven motors are energized by the digital computer to move the work piece rectilinearly along a selected axis. In addition, the welding is carried out within a controlled atmosphere and, in particular, the pressure and flow of gas into the welding chamber is controlled by the digital computer. Further, a counter is used to count pulses, whereby the number of laser pulses applied to the work piece may likewise be controlled.
U.S. Pat. No. 4,088,890 of Waters discloses a programmable controller for controlling laser emission and, in particular, the control of a high beam shutter whereby the desired quantity of laser emission is directed onto the work piece. This patent also discloses the rectilinear movement of a carriage carrying the work piece along a vertical axis, whereby the work piece is successfully brought to a position, where a laser weld is made. In particular, there is disclosed the effecting of a seam weld, whereby the work piece is rotated while the laser beam is directed at a seam between two pieces to be welded together.
U.S. Pat. No. 3,422,246 of Wetzel discloses a laser cutting machine tool including a servo system for controlling servo drive motors to drive a work piece along X and Y drive axes respectively. A transducer is associated with each of the servo motors to provide feedback signals indicative of the movement of the work piece along its respective axis to thereby ensure accurate work piece position.
In the initial development of laser machining systems, lasers were employed for individual, low production machining operations. With the development of the art, laser systems were increasingly employed for high production work processing operations as would be controlled automatically by computers. As described above, such high production systems operate efficiently to reposition the work piece, whereby a sequence of welds or other machining operations may be rapidly performed. Under such demands of continuing excitation, laser life becomes a factor in terms of efficient operation and of cost of production. It is contemplated that under high usage where repeated welds are required, as for the production of the above described fuel rod grids, that laser life would be a significant factor to consider. Under heavy usage, the life expectancy of the lamps exciting the pulsed laser would be in the order of several days, and after this life had been expended, it would be necessary to replace at least the lamps, as well as to calibrate the new laser system.
In order to improve laser efficiency and life, the prior art as illustrated by U.S. Pat. Nos. 4,223,201 and 4,223,202 of Peters et al. and U.S. Pat. No. 4,083,629 of Kocher et al., discloses the time sharing of a laser beam emitted from a single laser and alternatively directed along first and second optical paths onto a single work piece. U.S. Pat. No. 4,083,629 describes problems with automated welding systems wherein the work piece requires a plurality of welds to be made; in particular, the work piece may be brought to a first station, where a first welder is operated, and then transferred to a second station, whereat a second welder effects a welding operation. Alternatively, two welders could be used at a single station to effect the plurality of welds, thus minimizing the need to transport the work piece from the first to the second stations. However, these methods require that either the work piece be transported or reoriented, thus decreasing the production rate, or that two welders be used, thus substantially increasing the capital investment of such apparatus. As an attempt to overcome these problems, U.S. Pat. No. 4,083,629 suggests the use of a bimodal switching means, whereby the laser welder sequentially welds at two distinct weld sites. In particular, there is suggested a motor for rotating a reflected mirror, whereby the beam is alternately directed along a first and then a second focal path to the work piece to effect first and second welds on a single work piece such as an electrical component. It is apparent that there is an automated control of the laser welder to synchronize the firing of the laser with the switching of the laser beams, the wire cutting, and other handling operations. U.S. Pat. No. 4,223,201 describes a somewhat similar laser system adapted for larger work pieces such as would be encountered in ship construction. In particular, U.S. Pat. No. 4,223,201 suggests the use of a rotating mirror to sequentially direct the laser beam along first and second paths, whereby a single laser beam may be time shared. In addition, a suitable automatic controller is employed to control corresponding first and second welding heads that are moved in timed relationship with the beam sharing, so as to effect sequentially a series of welds at two different locations on a single work piece. U.S. Pat. No. 4,223,202 suggests the seam welding of two pieces together with the welding taking place on opposite sides of the work pieces at substantially the same point to effect a two sided laser seam weld, while the automated controller effects movement of the welding heads with respect to the work piece.
The above-referenced patent of Kocher et al. discloses the time shared control of a single laser source, whereby the laser beam may be split and directed along separate paths onto machining or weld sites. The Kocher et al. patent discloses that each of the two weld sites is disposed on a single work piece. Thus, there is no disclosure of the problems associated with the machining of two work pieces, where it is desired to time share control a single laser source and for directing its beam along separate paths onto the two distinct work pieces. In such a laser machining apparatus, it is typically desired to control the machining processes of each work piece. Each work piece is associated with its own work station and as contemplated by this invention, includes means for positioning each work station selectively in three dimensions with respect to the laser beam directed to that work station. In addition, it is contemplated that the material of which each work piece is made, is reactive to the normal atmosphere, and that it will be necessary to conduct the laser machining in an environment that is non-reactive to the work piece material. Thus, it is necessary to provide a sequence of control instructions to each work piece to not only effect the desired sequence of movements, but also to control the environment of each work station to ensure the production of high quality laser machining and in particular, welding.
The Kocher et al. patent also discloses laser welding apparatus and in particular suggests the use of a diverter mechanism disposed to intercept the laser beam, whereby the laser beam may be diverted into a heat sink. In particular, this patent suggests that the laser beam be diverted into a heat sink while the work piece is being replaced with another. Though such a technique permits the laser to keep firing at a uniform rate without being shut down so that its temperature, once established under equilibrium conditions, will not be altered between machining operations, this technique significantly reduces the efficiency of operation in that the laser machining and in particular the laser welding takes place for a relatively short period of time as compared to the time required to move or to replace the work piece.